Light-induced refractive index changes in low temperature glasses

ABSTRACT

Devices are made comprising a tin-phosphorous oxyfluoride glass, which has been exposed to light, preferably shorter in wavelength than the absorption edge of the glass, to change the refractive index change of the glass. The glasses can be used to form planar and fiber devices, including core/clad structures for guiding light.

This application is a 371 of PCT/US98/19963 filed Sep. 25, 1998whichclaims benefit of Prov. No. 60/060,845 filed Oct. 2, 1997.

FIELD OF THE INVENTION

The present invention relates to light-induced refractive index changesin materials. More particularly, the present invention relates to a lowtemperature glass exhibiting light-induced refractive index changes.

BACKGROUND OF THE INVENTION

The term “photorefraction” has been used to describe the phenomenonwhere the refractive index of a material is altered upon exposure tolight. Initially, the phenomenon was observed in a certain restrictedclass of crystalline materials that were photoconductive and exhibitedlarge polar effects. An example of such a material is LiNbO₃. There isalso a version of this type of “photorefractive” behavior that can beobtained in organic polymeric materials. In these polymeric materials,the various photosensitive agents are added to the polymer in aguest-host format.

More recently, a new class of materials have been reported to exhibitsignificant “photorefractive” behavior. These materials are glasses inthe family xSiO₂—(1−x)GeO₂. The origin of the effect in these materialsis totally different from that of the ferroelectric crystals mentionedabove. An excimer laser (193 nm and 248 nm) induces a refractive indexchange in these materials, and the refractive index change stems fromlarge absorption changes originating from “defects” in the glassstructure. Because the effect was originally discovered in a single modeoptical fiber and, since the major application of the induced indexchange has been to fabricate phase gratings in the fiber, this“photorefractive” behavior is often referred to as “fiber Bragggratings” in the technical literature. The effect has been reported tohave been extended to other binary SiO₂ compositions such as P₂O₅, SnO,and Ce₂O₃. By far the largest induced refractive index change has beenin the SiO₂—GeO₂ system where values as large as 0.001 have beenreported. The size of the refractive index effect may be increased inthe SiO₂—GeO₂ system by impregnating the glass with molecular hydrogenbefore exposure.

Tin-phosphorous oxyfluoride glasses are known and are disclosed in U.S.Pat. Nos. 4,314,031 and 4,379,070, which are relied upon andincorporated herein by reference. U.S. Pat. No. 4,314,031 discloses thattin-phosphorous oxyfluoride glasses desirably have a very low glasstransition temperature, frequently below 100° C., yet still exhibitexcellent resistance to attack by moisture at elevated temperatures.U.S. Pat. No. 4,379,070 discloses the use of tin-phosphorous oxyfluorideglasses as a matrix material for the support of photosensitive andelectric-field-responsive polycyclic aromatic hydrocarbon compounds.Neither of these patents, however, discloses or suggests thattin-phosphorous oxyfluoride glasses exhibit a “photorefractive” effector the use of tin-phosphorous oxyfluoride glass as a photorefractivematerial.

It would also be useful to provide a material that exhibits a“photorefractive” effect that could be doped with a variety of materialsfor altering the optical properties of the devices made from thematerial, including inorganic and organic dopants.

SUMMARY OF INVENTION

The present invention involves a tin-phosphorous oxyfluoride glasssystem that exhibits a large “photorefractive” effect by a mechanismdifferent from either of the crystalline or silica-germania classesmentioned above. It has been discovered that exposure of tin-phosphorousoxyfluoride glass to light of a wavelength shorter than the absorptionregion of the glass for a sufficient amount of time shows little or noabsorption changes, yet can exhibit refractive index changes greaterthan about 0.0004. A wide variety of devices may be fabricated fromtin-phosphorous oxyfluoride glass which has been exposed to light of awavelength shorter than the absorption region of the glass.

Accordingly, the present invention generally provides a device and amethod of making a device comprising tin-phosphorous oxyfluoride glasswhich has been exposed to light for a time sufficient to change therefractive index the glass. Preferably the wavelength of the light isshorter than about 350 nm, which roughly corresponds to the absorptionedge of the glass. The composition of the tin-phosphorous oxyfluorideglass may comprise, in weight percent on an elemental basis ascalculated from the batch, about 20-85% Sn, 2-20% P, 3-20% O, 9-36% F,and at least 60% total of Sn+P+O+F. The composition may further includeabout 0-40% cation modifiers and about 0-20% anion modifiers. Theglasses for making the devices of the present invention may further bedoped with a optically nonlinear organic dye to alter the opticalproperties of the glasses.

In one aspect of the invention, the device may include an opticalwaveguide region. In another aspect of the invention, the device mayinclude a periodic refractive index structure, such as a diffractiongrating. Accordingly, the devices of the present invention may be formedinto a variety of shapes, including planar and fiber forms. Gratings andwaveguides may be formed by changing the refractive index of selectedportions of the glass, which may be achieved by exposing the selectedportion to light shorter than the absorption edge of the glass. Forexample, an optical interference pattern may be utilized to form agrating, or the glass may be selectively masked to provide a periodicrefractive index structure in the glass.

Several important advantages will be appreciated from the foregoingsummary. One advantage of the device and method of the present inventionis providing a photorefractive device made from a glass material thathas a low glass transition temperature and is resistant to moisture. Thepresent invention also provides a photorefractive material that can bedoped with wide variety of materials such as optically nonlinear organicdies, which can be used to dynamically alter the optical properties ofthe devices. Another advantage is the ability to form photorefractivedevices into planar devices and fibers, including devices havinggratings and/or core/clad structures for guiding light.

Additional features and advantages of the invention will be set forth inthe description which follows. It is to be understood that both theforegoing general description and the following detailed description areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

DETAILED DESCRIPTION

Reference will now be made in detail to the present preferred embodimentof the invention. The present invention provides a device and a methodfor making the same.

The device of the present invention comprises tin-phosphorousoxyfluoride glass, which has been exposed to light for a time sufficientto change the refractive index change of the glass. The light ispreferably shorter in wavelength than 350 nm, which roughly correspondsto the absorption edge of the glass.

Reference is made to U.S. Pat. Nos. 4,314,031 and 4,379,070, for a moredetailed understanding of processing of tin-phosphorous oxyfluorideglass compositions. As disclosed in those patents, tin-phosphorousoxyfluoride glasses can be made from conventional batch materials suchas SnF₂, P₂O₅, Sn₃(PO₄)₂, SnO, NH₄H₂PO₄, NH₄PF₆ and Sn₂P₂O₇ and can bemelted at temperatures not exceeding 600° C. Preferably, however, toprovide glasses exhibiting the photorefractive effect, the compositionsshould be melted at temperatures below 450° C., and for somecompositions, as demonstrated by the example, preferably below 400° C.

As also noted in the patents, the tin-phosphorous oxyfluoride glasssystem may include a variety of additional optional constituentsincluding alkali metals, alkaline earth metals, group II metals such aszinc and cadmium, group II elements such as La, Ce, B and Al, group IVelements such as Pb, Zr, Ti, and Ge, group V elements such as Sb and Nb,group VI elements such as Mo and W, group VII elements such as Cl, Brand I, and group VIII metals such as Gd. Reference may be made to U.S.Pat. Nos. 4,314,031 and 4,379,070 for a further description of glasscompositions in the tin-phosphorous oxyfluoride composition system, mostof which are believed suitable for use in the present invention.

Glasses suitable for producing photorefractive devices may be preparedfrom commercial grade batch chemicals, melted in any suitable meltingunit, for example, a nickel, alumina, gold or vitreous carbon crucible,and formed by pressing, casting, blowing, molding, evaporation, drawing,and the like.

The composition of the tin-phosphorous oxyfluoride glass used to makethe devices of the present invention may comprise, in weight percent onan elemental basis as calculated from the batch, about 20-85% Sn, 2-20%P, 3-20% O, 9-36% F, and at least 60% total of Sn+P+O+F. Morepreferably, the composition of the glass used to make the devices of thepresent invention may comprise about 20-60% Sn, 9-20% P, 10-20% O, 9-20%F, and at least 60% total of Sn+P+O+F. The tin-phosphorous oxyfluorideglass may further include about 0-40% total of cation modifiers selectedin the indicated proportions from the group consisting of about up to12% Pb, up to 12% Zr, up to 5% Ca, up to 8% Ba, up to 5% Zn, up to 30%Tl, up to 7% Nb, up to 5% Ga, up to 5% Hg, and up to 4% total alkalis,such as Li, Na, K, Rb, and Cs. The glass composition may further includeup to about 0-20% total anion modifiers selected from the groupconsisting of Cl, Br and I.

The glass compositions may further be doped with optically nonlinearorganic dyes, such as acridine or rhodamine dyes. Such doping mayprovide optical property modifications, such as, for example, nonlinearoptical effects. The optically nonlinear organic dyes may be present inan amount of about 0.1%.

As mentioned above, the devices of the present invention comprisetin-phosphorous oxyfluoride glass which has been exposed to light,preferably shorter than about 350 nm, which generally corresponds to theabsorption edge of the glass, for a time sufficient change therefractive index of the glass. The exact wavelength selected to changethe refractive index will depend on the composition of the glass and theglass processing conditions, which may be determined by experimentation.The intensity of the light and duration of the exposure to the lightwill depend on the desired refractive index change, and to a lesserextent, the thickness of the glass.

In one aspect of the invention, the device of the present invention mayinclude a grating and/or a waveguide region such as a core/cladstructure. Accordingly, the devices of the present invention may befabricated into a variety of shapes, including planar and fiber forms.Thus, the glasses of the present invention may be utilized to fabricatea variety of devices such as optical fibers, lenses, planar waveguides,lightwave optical circuits, and a variety of planar devices.

A planar or fiber waveguide having core/clad structure for guiding lightmay be formed by exposing a portion of the glass to change therefractive index of the portion. For example, an optical fiber made fromtin-phosphorous oxyfluoride glass comprising a high index core regionsurrounded by a lower index cladding glass may be formed by changing therefractive index of the core and cladding regions. A lens may be formedby locally raising the refractive index of the core at the endface of anoptical fiber.

In another aspect, the devices of the present invention may include aperiodic refractive index structure, such as a diffraction grating.Gratings may be formed by a variety of techniques. For example, agrating structure may be formed by exposing a bulk piece of glass, inplanar or fiber form, to an optical interference pattern. It is knownthat an optical interference pattern may be formed by combining two ormore beams of light. Alternatively, a grating structure may be formed byselectively masking the glass of the present invention to selectivelyexpose a portion of the glass to the light utilized to change therefractive index of the glass.

A variety of compounds can be fabricated with the materials of thepresent invention, which could find application in such devicesincluding gratings, multiplexers, demultiplexers, filters and switches.A particular advantage of the photorefractive material of the presentinvention is the ease which the material can be doped with a variety ofmaterials to alter the optical properties of the glass including, forexample, organic dyes, as disclosed in 4,379,070.

The following non-limiting examples provide tin-phosphorous oxyfluorideglass compositions which, when exposed to light, exhibit refractiveindex changes and may be used to make the devices of the presentinvention.

EXAMPLE 1

A composition having the formula 56.3SnF₂—32.2P₂O₅—8.2PbO—3.4SnCl₂, inweight percent, was prepared according to the following procedure. Pbmetal was reacted with NH₄H₂PO₄ at 450° C. in an uncovered vitreouscarbon crucible until completion, which occurs when hydrogen ceases toevolve. This process took about 3 hours, and the reaction rate is about0.6 g/hr of Pb metal.

The crucible was removed from the furnace and momentarily cooled, thenthe remaining components (SnF₂ and SnCl₂. 2H₂O were added. The cruciblewas replaced in the furnace, covered and melted for one hour attemperatures between about 350° C. and about 400° C. The crucible wasremoved from the furnace and stirred under vacuum (about 28 inches ofmercury) for about five minutes at about 275° C. Rigorous foaming mayoccur at the start of this process, but when it did occur, it quicklysubsided and the melt became quite stable. At this point an organic dyecan be added to the glass if desired, as disclosed in U.S. Pat. No.4,379,070.

The melt was then transferred to a gold crucible, covered and replacedat the desired soak temperature for about 15 minutes. If the melts wereallowed to cool in vitreous carbon crucibles, instead of gold or othernonreactive crucible, then additional seeds were observed to form at theglass-carbon interface as the melt temperature decreases.

The glasses obtained from the above process contained very few seeds,but were optically nonhomogeneous because of the fluidity of the melts.The only significant shift in the starting composition was loss of about⅓ of the batched fluorine, which is stoichiometrically (2 for 1)replaced by oxygen. The cations and chlorine were essentially completelyretained.

The glass produced by the above process may be cast into disks, drawninto fibers, or pressed into thin sheets or films. Thin sheets may bemade by placing a small drop of the glass onto a preheated microscopeslide, silicon wafer or polished silica flat and then pressing the dropinto a thin sheet. For the above composition, the glass was pressed intothin sheets at a temperature near about 140° C. between two silicaplates. Generally, a temperature of about 75° to about 100° C. above theglass transition temperature is preferred for forming the thin sheets inthis manner. To avoid reaction of the hot glass with the substrate usedto press the sheets, the substrate may be coated with a non-stick,nonreactive surface such as a coating of gold or TiN.

The glass may then be exposed to a sufficient amount of activatingradiation, such as light, to induce a refractive index change in theglass. The activating radiation is preferably light shorter than 350 nm,which roughly corresponds to the absorption edge of the glass. A thinsheet made according to the example described above, was exposed to 309nm light from a XeCl laser for approximately 30 minutes through a maskto induce a refractive index change and develop a photorefractivepattern. The total flux of the laser was about 10 mJ/m² at a repetitionrate of about 50 Hz. The change in the refractive index of the glasswere directly measured by interferometry at 546 nm.

Gratings made from the composition in the above example lasted aboutfour months after exposure at room temperature without degradation ofthe index pattern. The refractive index pattern in the present examplewas a diffraction grating with a pitch of about 10 microns. Themagnitude of the refractive index change depended on the lasttemperature the glass was held at before cooling. For the abovecomposition, if the soak temperature was 400° C. or less, a refractiveindex change was observed after exposure to light shorter than theabsorption region of the glass. If the soak temperature was above 400°C., a refractive index change was not observed after exposure to light.However, if the temperature was raised above 400° C., and then loweredto and soaked at 400° C. or less, a refractive index change was observedafter exposure to light. Exposure to the 309 nm radiation alwaysdiscolored the glass to pale yellow, whether or not a refractive indexchange was observed.

A grating formed on sheet of glass made from the above composition andexposed to 309 nm radiation was measured with an interferometer using ameasurement wavelength of 546 nm. A fringe shift corresponding to achange in refractive index of at least 0.0002. The direction of thefringe shift indicated that the refractive index decreased in the areasexposed to the radiation. Glass of the above composition was alsoexposed to light of a wavelength of about 270 nm, and a refractive indexchange was also observed in the areas of the glass exposed to the light.

The composition in the above example was modified in an attempt toidentify the importance of each of the components. The modificationsconsisted of increasing and decreasing the cations, Pb and Sn by twoatomic percent, as well as increasing and decreasing the anions, F andCl, by two atomic percent. Raising the Cl content caused the glass tohave an opal appearance, but changing the anion content did notsignificantly effect the change in the refractive index after exposureto light. Increasing Sn decreased the glass transition temperature, butit did not significantly influence the magnitude of the refractive indexchange. Decreasing the cation percent of Pb decreased the magnitude ofthe refractive index change, and increasing the amount of Pb above 6atomic percent caused this glass to devitrify, forming a crystal phasewhich was identified as cotunnite (PbCl₂).

EXAMPLES 2-5

Each of the compositions below, expressed in weight percent, exhibited achange in refractive index when exposed to 309 nm light. Examples 3 and4 demonstrate that Pb can be substituted with another cation modifier,such as Zn.

2 3 4 5 Sn 48.5 47.7 22.8 47.9 P 12.1 14.5 14.3 10.7 Pb 7.1 0 0 10.3 Zn0 2.7 4.2 0 Tl 0 0 25.6 0 F 13.0 16.4 9.7 14.9 Cl 1.2 0 4.6 0.3 O 18.018.6 18.6 14.9

Tin-phosphorous oxyfluoride glass may be doped with an opticallynonlinear organic dye to modify the optical properties of the deviceproduced by the glass. About 0.1% of an optically nonlinear dye may beused to dope the glass. For example, doping tin-phosphorous oxyfluorideglass with a saturable absorber, such as acridine, will provide a glassin which transmission of light will depend on the intensity of light.The doped glass may be subsequently exposed to light to alter therefractive index of the glass to produce a variety of devices, such asan optically a periodic grating structure that is tunable depending onincident light intensity.

The change in refractive index effect in tin-phosphorous oxyfluorideglass after exposure to light shorter in wavelength than the absorptionedge of the glass is large enough to write gratings and waveguides. Thechange in refractive index is greater than 2×10⁻⁴ but based onmeasurements of grating efficiency, it is believed that the index changemay be greater than 10⁻³ , approaching 10².The higher refractive indexchanges were measured using an optical readout of the diffractionefficiency of a grating made from the glass using a HeNe laser at 633nm. The gratings made according to the above example could be erased byheating the glass above the glass transition temperature, which is about80° C. The magnitude of the refractive index change, coupled with thedemonstrated ability to introduce organic dyes makes this material anexciting optical material.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the device and method of thepresent invention without departing from the spirit or scope of theinvention. Thus, it is intended that the present invention cover themodifications and variations of this invention provided they come withinthe scope of the appended claims and their equivalents.

What is claimed is:
 1. A device comprising tin-phosphorous oxyfluorideglass which has been exposed to light for a time sufficient to changethe refractive index of the glass.
 2. The device of claim 1, wherein thelight is shorter in wavelength than the absorption edge of the glass. 3.The device of claim 2, wherein the tin-phosphorous oxyfluoride glass hasa composition which comprises, in weight percent on an elemental basisas calculated from the batch, about 20-85% Sn, 2-20% P, 3-20% O, 9-36%F, and at least 60% total of Sn+P+O+F.
 4. The device of claim 3, whereinthe tin-phosphorous oxyfluoride glass has a composition which comprises,in weight percent on an elemental basis as calculated from the batch,about 20-60% Sn, 9-20% P, 10-20% O, 9-20% F, and at least 60% total ofSn+P+O+F.
 5. The device of claim 4, wherein the tin-phosphorousoxyfluoride glass has a composition consisting essentially of, in weightpercent on an elemental basis as calculated from the batch, about 20-60%Sn; 9-20% P; 10-20% O; 9-20% F; 0-40% total cation modifiers selected inthe indicated proportions from the group consisting of about up to 12%Pb, up to 12% Zr, up to 5% Ca, up to 8% Ba, up to 5% Zn, up to 30% Ti,up to 7% Nb, up to 5% Ga, up to 5% Hg, and up to about 4% total alkalis;and up to about 20% total of anion modifiers selected from the groupconsisting of Cl, Br and I.
 6. The device of claim 4, wherein the lightis shorter than about 350 nm.
 7. The device of claim 6, wherein thedevice includes an optical waveguide region.
 8. The device of claim 6,wherein the device includes a diffraction grating.
 9. The device ofclaim 8, wherein the device is in planar form.
 10. The device of claim8, wherein the device is in fiber form.
 11. The device of claim 4further comprising an optically nonlinear organic dye dopant and aperiodic grating structure that is tunable depending on incident lightintensity.
 12. A method of making a device comprising the steps ofexposing a tin-phosphorous oxyfluoride glass to light for a timesufficient to induce a refractive index change in the glass.
 13. Themethod of claim 12, wherein the light is shorter in wavelength than theabsorption edge of the glass.
 14. The method of claim 13, wherein thetin- phosphorous oxyfluoride glass has a composition which comprises, inweight percent on an elemental basis as calculated from the batch, about20-60% Sn, 9-20% P, 10-20% O, 9-20% F, and at least 60% total ofSn+P+O+F.
 15. The method of claim 14, wherein the tin-phosphorousoxyfluoride glass has a composition consisting essentially of, in weightpercent on an elemental basis as calculated from the batch, about 20-60%Sn; 9-20% P; 10-20% O; 9-20% F; 0-35% total cation modifiers selected inthe indicated proportions from the group consisting of about up to 12%Pb, up to 12% Zr, up to 5% Ca, up to 8% Ba, up to 5% Zn, up to 30% Tl,up to 7% Nb, up to 5% Ga, up to 5% Hg, and up to about 4% total alkalis;and up to about 20% total of anion modifiers selected from the groupconsisting of Cl, Br and I.
 16. The method of claim 14, wherein thelight is shorter than about 350 nm.
 17. The method of claim 13, furthercomprising the step of exposing the glass to an optical interferencepattern.
 18. The method of claim 13 further comprising the step ofselectively masking the glass to produce a periodic pattern in theglass.
 19. The method of claim 13 further comprising the step of formingan optical waveguide structure comprising a core and a cladding.
 20. Themethod of claim 19 further comprising the step of forming the glass intoa fiber.